1. Field of the Invention
The present invention relates to photographic silver halide films which undergo tanning development, and in particular deals with photographic silver halide films of lowered silver halide content in which the silver density is enhanced by dye mordanting development.
2. Related Prior Art
When gelatin, or leather, is tanned cross-links are formed between the long protein molecules, which are thus rendered insoluble and mechanically durable. On this principle, the function of a tanning developer in the field of photography is to develop the latent image in an exposed gelatino-silver halide emulsion under conditions such that the oxidation products from the developer form bridges between gelatin molecules in the vicinity thereof.
There is a close analogy between tanning development and color development, and in some instances an apparent identity. Image production by the reaction of a positively charged developer oxidation product and a negatively charged color coupler which unite to form a dyestuff is well known in color photography, and in some cases is accompanied by tanning of the gelatin in image areas. Modifications of the color production process in diffusion transfer systems has involved mordanting an acid dye image in a layer as in Whitmore U.S. Pat. No. 3,271,148. Other patents involving developer and dye combinations in photographic films are U.S. Pat. Nos. 3,887,563; 3,817,751; 3,728,113; 3,622,629; 3,590,692; and 3,243,294.
A common characteristic of these prior art teachings is that a special film is required, the chemical structure of which participates in the reaction to produce the desired image density. They do not contemplate attaining such density with conventional films which dispense with special additions to the silver halide emulsion. In this respect the present invention departs from the prior art.